Connections between electrical conductors and carbon bodies and method of making same



0a. 27, 1970 M. w. VQEL'KER 3,536,460

CONNECTIONS BETWEEN ELECTRICAL CONDUCTORS AND CARBON BODIES AND METHODOF MAKING SAME Filed Dec. 28. 1966 Calida/(for lk vped Carbon UnitedStates Patent Oflice 3,536,466 Patented Get. 27, 1970 3,536,460CONNECTlONS BETWEEN ELECTRICAL CON- DUCTORS AND CARBON BODIES AND METHODOF MAKING SAME Marvin W. Voelker, Buffalo, N.Y., assignor to Great LakesCarbon Corporation, New York, N.Y., a corporation of Delaware Filed Dec.28, 1966, Ser. No. 605,285 Int. Cl. B321!) 33/00 US. Cl. 29-195 14Claims ABSTRACT OF THE DISCLOSURE A low electrical resistance jointassembly is made between a carbon body and an electrical conductor. Oneof these assembly members contains an internally threaded recess and theother is externally threaded and the two members are in threadedengagement. An alloy is within the recess between threads of the carbonbody and threads of the conductor. The alloy has been heated above itsmelting point, and is then cooled to solidify it between threads of theassembly members. Threaded engagement between the conductor and thecarbon body is effected while the alloy is in a molten condition withinthe recess.

The alloy expands or exhibits cumulative growth during and aftersolidification, thus adding to the mechanical bond and engagementbetween the threaded carbon body and the threaded electrical conductorand thus forming a connection of low electrical resistance and goodstrength.

Typically the carbon body may be a graphite lead-in rod, the electricalconductor a copper rod, and the connections made are for use inelectrolytic cells.

This invention relates most particularly to graphite pins for mercurycell anode plates and end connector plates for chlorate cells, andjoints between such pins and end connector plates and copper bars. Morebroadly, the invention relates to unique connections between anelectrical conductor and a carbon body which can be used for anypurpose, such as in electrolytic cells, or elsewhere, and to methods ofpreparing same.

Several designs of mercury-cathode chlor/alkali cells use graphite pinsor stems to support the anode plates and carry the operating currentthrough the cell cover from the external current source to the anodeplate. Such pins are illustrated in the accompanying drawing. These pins3 typically have a hole or recess drilled in their center and a copperbar or rod 6 fixed in this hole and extending above the top end of thegraphite pin. In such a typical construction, the diameter of the copperbar is less than the diameter of the hole in the graphite and theannular space between the copper and graphite is filled with anelectrically conducting material by means of which a connection is madebetween the copper and graphite. This connection is intended to bemechanically strong and to have a low electrical resistance. The end ofthe pin 3 is used for coupling to the anode plate in order to supportsaid plate. This connection is typically made either by press-fitting orthreading the pin 3 into a mating cylindrical or threaded hole in thetop of the anode plate.

An end connector plate for chlorate cells, also typically made ofgraphite, may employ the same type connection and have a hole drilledthrough most of its length near one edge. A copper bar is fixed in thishole and the end of the bar extends above the top of the graphite plate.The diameter of the copper bar is smaller than the diameter of the holein the graphite plate, and, as in the case of the graphite pins justdescribed, this annular space between the copper and the graphite isfilled with an electrically conducting material by means of which aconnecnection is made between the copper and graphite. This connectionis also intended to be mechanically strong and to have a low electricalresistance. The end connector plates are typically reinforced in thearea of the drilled hole by graphite reinforcing strips cemented to theplate on the opposing sides. Alternatively, they are reinforced byhaving been machined with a thicker cross-section near the edge and inthe region wherein the hole is located.

The present invention is chiefly concerned with and has as one of itsobjects the making of a low-electrical resistance, high-strength jointbetween an electrical conductor, such as a copper or aluminum or steelor carbon rod, and a graphite pin or plate which can conveniently beused as an electrical coupling to an anode of an electrolytic cell;and/or to the making of such joints in end connector plates for chloratecells. The invention, however, is applicable to any such low-electricalresistance, high-strength joint made according to the teachings of thisinvention, no matter where or how it is used. Such joints made accordingto the present invention offer several advantages over jointconstruction utilized in the past, such as substantially reduced voltagedrop through the joint, increased joint strength, and substantiallyuniform electrical resistance for prolonged periods of time. Furtherother objects and advantages will become apparent after a review of thedescription of the present invention, and the comparison of same withother practices in the art.

It is a finding of the present invention that an electrical conductor,such as a copper rod, may conveniently and advantageously be coupled toa lead-in rod, graphite pin, or carbon body by means of the combinationof a threaded engagement and an expanding alloy between the electricalconductor and the carbon body. The alloy is heated to a temperatureabove its melting point and is then permitted to cool below its meltingpoint within a threaded recess in one of the members to be joined, intowhich the other member (which is also threaded) is threaded while thealloy is molten. The alloy is characterized byits exhibiting acumulative growth during and after solidification. Thus a tightmechanical bond as well as a threaded engagement is developed betweenthe carbon body and the electrical conductor, forming a low resistanceelectrical connection. The alloy is further characterized by having amelting point between about C. and about 300 C. The alloy also typicallypossesses an electrical resistivity between about 0.S 10" and about 5.O10 ohm-inches and preferably no higher than l0 10* ohm-inches. It ispreferable, although not necessary, that the alloy wet or bond to thethreaded electrical conductor (e.g. to the threaded copper rod if thatis what is used).

There are many alloys which fulfill the foregoing requisites and whichcan be employed in the invention. The following compositions, whereinthe numbers are approximate percentages by weight, and having theproperties set forth opposite same, are typical or preferred alloyswhich may be used:

Properties Eleetrica l resistivity of alloy in ohm-inches Meltingtemperature or range, C.

Alloy Because of the fact that all of the foregoing alloys exhibit acumulative growth during and after solidification (as a result ofexpansion either upon being cooled from their molten state to theirsolid state, and/or because of linear growth after solidification), avery tight joint or connection is made between the electrical conductorand the carbon body. The joint so made is also very strong mechanicallyand has a very low electrical resistance when any of these alloys isused in making the threaded connection as described herein.

Before the connection is made, and with reference to the particularconstruction illustrated in the drawing but not restricted thereto, theelectrical conductor or copper bar or rod is prepared for use bythreading a portion of its length such as illustrated in the drawing,and also generally by mechanical and/or chemical cleaning. The carbonbody or graphite stem or end connector plate is correspondinglythreaded, by tapping, and is also generally prepared by blowing thethreaded recess out with air pressure or otherwise cleaning it. Theexternal threads of the electrical conductor and the internal threads ofthe recess are so designed that the threaded engagement is not so tightthat it is impossible for the alloy to be forced between the matingthreads when the conductor is threaded into the recess.

Unified screw thread standards such as set forth in Tables 1-7 forclasses 1A, 1B, 2A, 2B, 3A and 3B in Order 50-7 from the NBS Circular479; (Supplement to NBS Handbook H28; and A.S.A. Bl.ll949all dated 1949)are indicative of the threaded arrangements which may be used for theelectrical conductor and carbon body employed in the present invention.Typically a standard thread size appropriate for the diameter of theconductor will be used. The conductors used in the present inventionwill typically be copper having a diameter between about /2 inch andabout 4 inches for most of the uses contemplated in using theconnections of the present invention. The connection is made by aprocedure such as now described.

In one method, (illustrated by using copper as the electrical conductorand graphite as the carbon body) both the externally threaded copper baror rod and the internally threaded graphite stem or plate are preheatedto some convenient temperature above the melting point of the alloybeing used. A quantity of molten alloy sufficient to fill the spacebetween the threads and to rise to the top of the connection is thenpoured into the hole in the graphite and the copper bar is threaded intothe hole, typically until it is hand-tight. As the copper bar approachesthe bottom of the hole and reaches the molten alloy, the alloy 4 isforced to rise around the copper bar and to distribute itself betweenthe threads of the conductor and the graphite. Typically, and asillustrated in the drawing, the alloy 4 will be between the bottom sidesof the threads of the copper conductor and the upper sides of thethreads of the graphite, with direct contact between the upper sides ofthe threads of the copper and the lower sides of the threads of thegraphite. Alloy 4 may also be found between the bottom of the copper bar6 and the bottom of the recess. Any excess alloy 4 forced to the top ofthe connection can be poured or brushed off. The assembly is thenallowed to cool to a temperature below the freezing or solidificationpoint of the alloy.

Alternative procedures may also be used which would involve heating thecopper conductor and graphite body to temperatures other than above themelting point of the alloy.

In another method, a pellet (or pellets) of alloy (or alloy powder)sutficient to fill the space between the threads and to rise to the topof the connection when melted is placed in the bottom of a threaded andcleaned recess of a graphite stem or end connector plate. The threadedand cleaned copper bar, prepared as before, is threaded into thethreaded recess in the graphite to contact the top of the pellet (orpowder) of alloy, all ma terials being then at room temperature. Theassembly is then heated uniformly by a suitable method to a temperaturein excess of the melting point of the alloy. The copper bar is thenthreaded further into the threaded recess in the graphite until it isapproximately hand-tight and thus forces the molten alloy to rise and tofill the space between the threads. Any excess alloy is poured off as inthe first assembly method. The assembly is then cooled to freeze orsolidify the alloy.

Advantages of the type of copper to graphite or electrical conductor tocarbon body connection made by the methods disclosed in this inventionare: the joints are tight and mechanically strong; the joints have lowerelectrical resistance than joints made with solders containing tin andlead only which do not expand upon cooling; the joints also have lowerelectrical resistance than connections made by means of threads alone orby means of an expandable alloy alone; the electrical resistance of thejoints is substantially stable for prolonged periods of time; the jointsare easy to assemble and free from susceptibility to vibration andhandling effects; in addition, if the assembly is employed in anapplication where inadvertently the temperature rises above the meltingpoint of the alloy, the conductor and the carbon body will remain joinedby the threads. Further, the low-resistance nature of the joint will beire-established automatically upon cooling of the assembly below themelting point of the alloy.

The following examples further illustrate the invention:

EXAMPLE 1 Two assemblies were prepared according to the aforedescribedfirst method and as illustrated in the drawing. the assemblies were madeusing graphite pins 2 /2 inches in diameter and 7% inches long as thecarbon body, copper 1 inch in diameter and 14% inches long as theelectrical conductor, and 58 bismuth, 42 tin as the alloy. In bothassemblies, 5 /sinches of one end of the flat bottomed copper rods werethreaded in accordance with specifications of the aforedescribed UnifiedScrew Thread Standards for 1 inch diameter, 8 threads per inch, UNO- 2A.One of the graphite pins was bored and tapped for 1 inch-8 threads perinch, UNC-ZB to a depth of 5% inches. The other graphite pin was boredand tapped for 1 /3 inches-8 threads per inch, UNC-2B to a depth of 5%inches. Both graphite pins were reamed to 1 /8 inch diameter for anaddition inch making the overall hole depth 5% inches. The hole bottomswere also flat. Joint resistances were measured by passing a knowncurrent through the completed assemblies via electrical contactsattached to the extreme ends of the assemblies. Voltage drop across thejoints was measured by attaching radial contacts around the copper rodsjust above the point where the copper emerges from the graphite andaround the graphite pin at a point just below the end of the copper rod.The null balance method of measuring voltage drop was used to avoid theeffects of contact resistance. The results of these tests demonstratedthat the joint resistances as measured by the voltage drops across thejoints were very low; and that the resistances and voltage drops weresubstantiallly lower than those of joints made with solders containingtin and lead only which do not expand upon cooling, or than those ofjoints made by means of threads alone, or by means of an expandablealloy alone (viz without the added feature of the threaded engagement).The assemblies were then heated in air to C. for a period of 10 days,cooled to room temperature and their voltage drops were measured again.They were tested in this manner for days with no significant change inthe originally low resistances and/ or voltage drops.

Several anode stems were also assembled according to the aforedescribedfirst method and using the alloys b-e. Results similar to those ofExample 1 were obtained. Very satisfacotry results were also obtainedusing the alloys 0-8 in the other assembly method previously described.

In use in an electrolytic cell, the lead-in rod assembly and the anodeand the connection between same are seldom heated to temperatures above8090 C. Therefore, when used in an electrolytic cell, the alloy used tomake the joint is not generally heated to a temperature above about 95C. nor are such temperatures at all desirable or necessary. Because theconnections made in the present invention include the heating of thealloy up to elevated temperatures so as to cause the alloy to melt andthen solidify upon cooling, it is preferred that for any given use andfor optimum results the joint be used below the melting point of theparticular alloy used. As illustrated by the properties set forth of thealloys a-e, the melting point or range of the alloys used in the presentinvention vary from about 103 C. to about 280 C., or slightly less thanabout 300 C. However, as previously pointed out, because the joint isthreaded an inadvertent temperature rise above the melting point of thealloy will not destroy the connection, and when the temperature is againlowered below this point the joint is substantially as good as it wasinitially.

The joints of this invention are easily adaptable to lead-in pins of thethreaded or non-threaded (e.g. pressed-fit) type, i.e. with respect tothe portion of the pin which is coupled to the anode, and can easily beprepared by the manufacturer of the anodes and/or the manufacturer ofthe lead-in pins as well as by the cell operators. In other words, ifdesired, the lead-in assembly of the copper rod and graphite pin can beprepared in advance and shipped to the cell operator and does not haveto be prepared at the site of the cell. They (the cell operators) canalso prepare the present joints.

The low resistance joints or connections of the present invention can beused in arts or fields other than the electrolytic cell art, for examplein the carbon brush field, or in making a low-resistance couplingbetween any electrical conductor and a carbon body. Also, in thiscontext, the term carbon body is intended to connote or include eitherbaked (or amorphous) or semigraphitic or graphitized carbon bodies,impregnated or unimpregnated, which bodies also can vary greatly intheir size and shape, density and porosity, and ultimate intended use.In certain instances, or for certain applications, the carbon bodies ofthis invention may also include minor amounts of materials other thanstrictly carbon or graphite, such as sintered metals. All such joints orconnections, however, are characterized by having a threaded recess inone of the members to be joined, which recess is adapted for thethreaded engagement of a matching threaded electrical conductor thereinand also for the hereindescribed alloys therebetween. Typically thethreaded recess will be in the carbon body, which will thus beinternally threaded, and the electrical conductor will be externallythreaded. However, this may be reversed and the electrical conductor maycontain the threaded recess and be internally threaded and the carbonbody may be the male, externally threaded member. The threaded recesswill typically and preferably be cylindrical, although it may also haveother shapes such as a threaded truncated cone. It is obvious, ofcourse, that the recess must terminate within the member containingsame, rather than extend all the way through, in order that the methodsof making the connections of this invention may be carried out. Its sizeand shape and depth will be dependent upon the size and shape of thecarbon body and/or of the electrical conductor. As aforesaid, however,the recess will most preferably be cylindrical and the externallythreaded male member to be inserted into same will also be cylindrical,with a slight clearance therebetween such as provided by variousstandard thread gauge systems used, i.e. NC, UNC, etc., so as to permitthe forcing of the molten alloy between the threads and up out of therecess as the male member is threaded into the female member. In anycase, however, the joint will be effected by a threaded engagementbetween the electrical conductor and the carbon body and also by meltedalloys which exhibit a cumulative growth during and after solidificationas previously described. The space between the threads of the electricalconductor and threads of the carbon body may be varied and will bedependent somewhat upon the depth of the threaded recess in the femalemember and the degree of penetration of the threaded male member intosame and the strength of the joint required and the purpose or end usefor the assembly. It should be noted, however, that while this variablespacing between the threads is generally kept to a minimum to reduceoverall joint resistance, there must be sufiicient clearance to permitthe molten alloy to be forced therebetween in order to accomplish thepurposes of the invention.

Having thus described the nature of my invention and the uses for same,but being limited only by the appended claims with respect to the scopeof the invention, I claim:

1. A joint assembly comprising a carbon body and an electricalconduction one of which contains an internally threaded recess and theother of which is externally threaded, and both being in threadedengagement, said assembly including an alloy within said recess betweenthreads of the conductor and threads of the carbon body, said alloyhaving been heated to a temperature above its melting point and whichalloy has then been permitted to cool below its melting point withinsaid threaded recess after the threaded engagement between the conductorand the carbon body has been effected, said alloy exhibiting acumulative growth during and after solidification thus adding to themechanical bond and engagement between the threaded carbon body and thethreaded electrical conductor and thus forming a low resistanceelectrical connection, said alloy also having a melting point betweenabout C. and about 300 C.

2. The joint assembly of a carbon body, an electrical conductor, and analloy according. to claim 1 wherein said alloy possesses an electricalresistivity no higher than 10 10 ohm-inches.

3. The joint assembly of a carbon body, an electrical conductor, and analloy according to claim 1 wherein the electrical conductor is anexternally threaded copper rod, wherein the carbon body contains theinternally threaded recess, and wherein the carbon body is graphite.

4. The joint assembly of a carbon body, an electrical conductor, and analloy according to claim 3 wherein the graphite body is a lead-in rodfor use in electrolytic cells.

5. A joint assembly comprising a carbon body and an electrical conductorone of which contains an internally threaded recess and the other ofwhich is externally threaded, and both being in threaded engagement,said assembly including an alloy within said recess between threads ofthe conductor and threads of the carbon body, said alloy having beenheated to a temperature above its melting point and which alloy has thenbeen permitted to cool below its melting point within said threadedrecess after the threaded engagement between the conductor and thecarbon body has been effected, said alloy exhibiting a cumulative growthduring and after solidification thus adding to the mechanical bond andengagement between the threaded carbon body and the threaded electricalconductor and thus forming a low resistance electrical con nection, andsaid alloy being selected from the group consisting of the followingcompositions wherein the numbers are approximate percentages by weight:

58 bismuth, 42 tin;

55.5 bismuth, 44.5 lead;

48 bismuth, 28.5 lead, 14.5 tin, 9 antimony;

15 antimony, 58 lead, 26 tin, 1 copper; and

15 antimony, 82 lead, 3 tin.

6. The joint assembly of a carbon body, an electrical conductor, and analloy according to claim wherein the alloy used is 5 8 bismuth, 42 tin.

7. The joint assembly of a carbon body, an electrical conductor, and analloy according to claim 5 wherein the alloy used is 55.5 bismuth, 44.5lead.

8. The joint assembly of a carbon body; an electrical conductor, and analloy according to claim 5 wherein the alloy used is 48 bismuth, 28.5lead, 14.5 tin, 9 antimony.

9. The joint assembly of a carbon body, an electrical conductor, and analloy according to claim 5 wherein the alloy used is 15 antimony, 58lead, 26 tin, 1 copper.

10. The joint assembly of a carbon body, an electrical conductor, and analloy according to claim 5 wherein the alloy used is 15 antimony, 82lead, 3 tin.

11. The joint assembly of a carbon body, an electrical conductor, and analloy according to claim 5 wherein the electrical conductor is anexternally threaded copper rod, wherein the carbon body contains theinternally threaded recess, and wherein the carbon body is graphite.

12. The joint assembly of a carbon body, an electrical conductor, and analloy according to claim 11 wherein the graphite body is a lead-in rodfor use in electrolytic cells.

13. The joint assembly of a carbon body, an electrical conductor, and analloy according to claim 1 wherein the connected threaded carbon bodyand electrical conductor are used at temperatures below the meltingpoint of the alloy.

14. The joint assembly of a carbon body, an electrical conductor, and analloy according to claim 5 wherein the connected threaded carbon bodyand electrical conductor are used at temperatures below the meltingpoint of the alloy.

References Cited UNITED STATES PATENTS 3,048,434 8/1962 Johnson.3,055,789 9/1962 Gemmi. 3,399,322 8/1968 Ambe.

HYLAND BIZOT, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent z 'na aenDated (kfnhF-r 77 iq'm Inv n fl Marvin m lfnp'lkpr It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 2, line 22, delete "construction" and insert-- constructions.

In Table I, line 69, re alloy d., under the "Electrical Resistivity"column, delete "9.82 x 10 and insert-- Claim 1, line 2, delete"conduction" and insert--conductor--.

E'JLRLED FEBe 19M FORM PO-105O (10-69) USCOMM-DC wan-Pan I 05 GOVIRNIIH"IIINYING OFFICE: "I. lJ-lii-lll

